Label-free sensing methods have the potential to enable detection and characterization of chemical species at the single-molecule level without the aid of probes or modifications to the target of interest. Such a capability would have broad applicability to medical diagnostics, toxicology, and chemical quantification. Label-free imaging strategies offer additional possibilities in the context of revealing organization of biological and materials systems over a range of length scales. Whispering gallery mode (WGM) optical microresonators have been shown to be a highly effective platform for label-free sensing, reaching the single protein and single nucleotide level, and enabling label-free imaging of non-emissive particles. Light is evanescently coupled into WGM resonators at specific resonant wavelengths and will circulate many times, allowing the mode to repeatedly sample the surface of the resonator, making these devices highly sensitive to changes in microenvironment. Additionally, the surface can be functionalized with receptors, antibodies, DNA complements, etc, to enable chemical selectivity.
While a variety of microresonators—including microspheres, microrings, nanofibers, photonic crystal cavities and Fabry-Perot resonators—have been used for sensing applications, microtoroid and microsphere resonators have shown the highest levels of sensitivity, including reaching the single molecule limit. This success derives from two superior properties: their high quality (Q) factor and low mode volume (V), with the ratio Q/V ratio dictating the sensitivity. Microtoroid resonators possess the additional advantages of on-chip fabrication, a critical requirement for large-scale fabrication, and a mode spectrum of substantially reduced complexity. The lithographic fabrication of SiO2-on-Si microtoroids takes advantage of material contrast to execute the fabrication: a silica resonator atop a silicon pillar and substrate. However, fabrication of an oxide microresonator on a silicon pillar uses a highly stochastic laser-induced reflow step for smoothing the rim and limiting scattering losses. The laser reflow process must be conducted on each individual microtoroid, introducing complexity and heterogeneity into the process and ultimately preventing wafer-scale fabrication.
It has been shown that the sensitivity of microtoroid resonators to their microenvironment can be exploited to enable a platform for label-free single-particle microscopy. Small changes to the refractive index at the surface of the microresonator result in a shift of the resonant frequency. Because changes in the refractive index are proportional to the local temperature, the resonance shift can be used as an indicator of local temperature change at a point on the surface of the resonator. Microresonators are thus extraordinarily sensitive thermometers, including quantification of the thermal energy dissipated from a relaxing molecule after optical excitation. Microtoroids are a highly suitable platform for conducting these measurements because the Q/V ratio minimizes the volume that needs to be heated, as well as provides the ultra-narrow linewidth required to distinguish minute resonance shifts.